Platinum catalyzed 7-endo cyclization of internal alkynyl amides and its application to synthesis of the caprazamycin core

The scope and limitations of the platinum catalyzed 7-endo cyclization of internal alkynyl amides were investigated. Substitution of the alkyne with an aryl group gave better results, presumably because it stabilized the transition state. Applying the reaction to a secondary amide, the caprazamycin core was successfully synthesized from commercially available material in eight steps.     

Nonparametric Bayesian drift estimation for multidimensional stochastic differential equations*

We consider nonparametric Bayesian estimation of the drift coefficient of a multidimensional stochastic differential equation from discrete-time observations on the solution of this equation. Under suitable regularity conditions, we establish posterior consistency in this context.     

Efficient Synthesis of a Regioregular Oligothiophene Photovoltaic Dye Molecule,MK‐2, and Related Compounds: A Cooperative Hypervalent Iodine and Metal‐Catalyzed Synthetic Route

We have successfully established an efficient route to the core structure of donor–acceptor head‐to‐tail (H–T)‐linked regioregular oligothiophenes, which includes the following key synthetic steps, that is, hypervalent iodine induced direct and regioselective coupling of thiophenes and the use of the obtained bithiophenes as excellent coupling substrates for the Suzuki and Stille couplings. The versatility of this new approach is highlighted in the dramatic improvement of the yield (ca. 59 % overall yield) of MK‐2, a high‐performance organic dye, for photovoltaic applications.     

Complete thermal-unfolding profiles of oxidized and reduced cytochromes C

The complete thermal-unfolding profiles of both oxidized and reduced forms of cytochrome c551 (PA) from mesophilic Pseudomonas aeruginosa and cytochrome c552 (HT) from thermophilic Hydrogenobacter thermophilus were obtained by the newly developed pressure-proof cell compartment installed in a circular dichroic spectrometer, which facilitates protein thermal-unfolding experiments up to 180 degrees C. The thermodynamic cycle, which relates protein stability and redox function, indicated that the redox potentials of PA and HT in the native state are regulated by the stability of the oxidized proteins rather than by that of the reduced ones.     

Cleavage of a P=P Double Bond Mediated by N-Heterocyclic Carbenes

The reaction of the bulky diphosphenes (Rind)P=P(Rind) ( 1 ; Rind=1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl) with two molecules of N-heterocyclic carbene (NHC; 1,3,4,5-tetramethylimidazol-2-ylidene) resulted in the quantitative formation of the NHC-bound phosphinidenes NHC→P(Rind) ( 2 ), along with the cleavage of the P=P double bond. The reaction times are dependent on the steric size of the Rind groups (11 days for 2 a (R=Et) and 2 h for 2 b (R=Et, Me) at room temperature). The mechanism for the double bond-breaking is proposed to proceed via the formation of the NHC-coordinated, highly polarized diphospehenes 3 as an intermediate. Approach of a second NHC to 3 induces P−P bond cleavage and P−C bond formation, which proceeds through a transition state with a large negative Gibbs energy change to afford the two molecules of 2 , thus being the rate-determining step of the overall reaction with the activation barriers of 80.4 for 2 a and 29.1 kJ mol⁻¹ for 2 b .     

Impact of Group 10 Metals on the Solvent-Induced Disproportionation of o-Semiquinonato Complexes

The oxidation of [M^II(3,5-DTBCat)(DTBbpy)] (M=Ni ( [Ni] ), Pd ( [Pd] ), and Pt ( [Pt] ); 3,5-DTBCat=3,5-di-tert-butylcatecholato; DTBbpy=4,4′-di-tert-butyl-2,2′-bipyridine) afforded the dimeric {[Ni^II(3,5-DTBSQ)(DTBbpy)](PF₆)}₂ ( {[Ni](PF₆)}₂ ; 3,5-DTBSQ=3,5-di-tert-butylsemiquinonato) and monomeric semiquinonato (SQ) complexes [M^II(3,5-DTBSQ)(DTBbpy)](PF₆) (M=Pd ( [Pd](PF₆) ) and Pt ( [Pt](PF₆) )). The negative solvatochromic properties of the SQ complexes allowed us to estimate the relative order of their dipole moments: [Pd](PF₆) > [Pt](PF₆) > {[Ni](PF₆)}₂ . The complexes [Pd](PF₆) and [Pt](PF₆) adopt monomeric structures and are stable in CH₂Cl₂ and toluene, whereas they gradually disproportionate at room temperature to [M] and 3,5-di-tert-butylbenzoquinone (3,5-DTBBQ) in polar solvents such as THF, MeOH, EtOH, DMF, or DMSO. The results of spectroscopic studies suggested that the oxidized nickel complex adopts a monomeric structure ( [Ni](PF₆) ) in CH₂Cl₂, but a dimeric structure ( {[Ni](PF₆)}₂ ) in the other investigated solvents. In polar solvents, {[Ni](PF₆)}₂ may disproportionate to [Ni] and 3,5-DTBBQ at 323 K, thereby demonstrating a significant solvent- and metal-dependence in temperature. The relative activities of {[Ni](PF₆)}₂ and [M](PF₆) toward disproportionation are related to the electrochemically estimated K_dis values in CH₂Cl₂ and DMF. The present work demonstrates that solvent polarity and the dipole moments of the SQ complexes promote disproportionation, which can be controlled by a judicious choice of the metal ion, solvent, and temperature.     

Photochemistry of naphthalimide photoacid generators

The photophysical properties of a series of 1,8-naphthalimide photoacid generators were studied by steady state fluorescence and phosphorescence spectroscopy. Emission and excitation anisotropies, triplet quantum yields in polar and nonpolar solvent and photoacid generation were evaluated. The singlet excited state exhibits a low polarity and is strongly deactivated by an efficient intersystem crossing process. In protic solvent, a homolytic singlet cleavage of the N-O bond occurs and leads to the acid production. The existence of a triplet state close to the singlet state was clearly evidenced. The presence of close singlet excited states is supported by fluorescence anisotropy and picosecond laser spectroscopy experiments. Results of DFT calculations well confirm the experimental contentions and yield important information about the cleavage process involved in such compounds.     

Carbon- and nitrogen-centered radicals produced from l-lysine by radiation-induced oxidation: A pulse radiolysis study

Radical species generated from the reactions of a basic amino acid, l-lysine (Lys), with hydroxyl radicals (OH) and sulfate radical anion () have been detected by the method of pulse radiolysis. On the basis of electron transfer reactivities toward tetranitromethane (TNM), it was demonstrated that reducing carbon-centered radicals are generated as a result of hydrogen abstraction from CH₂ of Lys with a G-value of 1.9 × 10⁻⁷ mol J⁻¹. On the other hand, direct oxidation of l-Lys by formed a transient species with different spectroscopic properties, most likely, the ε-N-centered Lys radical.     

A TEMPO-conjugated fluorescent probe for monitoring mitochondrial redox reactions

We report a mitochondrial targeted redox probe (MitoRP) that comprises a nitroxide radical (TEMPO) moiety and coumarin 343. Using isolated mitochondria in the presence/absence of substrates and inhibitors of oxidative phosphorylation, we demonstrated that MitoRP is a useful probe to monitor the electron flow associated with complex I.